There has been an antenna, in which a conductor is patterned and formed on the surface of a circuit board, for the purpose of reducing the size. As this type of antenna, for example, a so-called inverted F-type antenna has been widely known. JP-A-2004-56506 describes an inverted F-type antenna formed on a dielectric substrate 21.
The inverted F-type antenna described in the publication has such a structure in which a radiation electrode 22 is formed by releasing one end of a conductor film formed from one surface of the dielectric substrate 21 to one side and connecting the other end part of the side to the ground electrode 23 provided on the rear side, and a feeding pin 24 is connected to a feeding point 22a located at a side nearer to the connection end with the ground electrode 23 via through holes in the dielectric substrate 21 and the ground electrode 23.
The resonance frequency of the inverted F-type antenna is determined by the length of the radiation electrode 22 and the dielectric constant of the dielectric substrate 21. That is, as the radiation electrode 22 becomes longer, the resonance frequency becomes lower. If the length of the radiation electrode 22 is the same, as the dielectric constant of the dielectric substrate 21 becomes larger, the resonance frequency becomes lower. In addition, it is known that the resonance frequency of the inverted F-type antenna is varied even by the length between the connection end of the radiation electrode 22 with the ground electrode 23 and the feeding point 22a. The resonance frequency becomes lower as the length between the connection end of the radiation electrode 22 with the ground electrode 23 and the feeding point 22a becomes shorter.
Meanwhile, a conductor pattern of an antenna designed for one circuit board is desired to be used for another circuit board. However, as described above, since the resonance frequency of the inverted F-type antenna described in the publication is determined by the dielectric constant of the dielectric substrate 21, the resonance frequency of the antenna is varied if the same conductor pattern is used for a circuit board (dielectric substrate) having a different dielectric constant. Therefore, in order to obtain a desired resonance frequency in different circuit boards, there is a problem that the conductor pattern of an antenna is designed for each of the circuit boards according to the dielectric constant of the circuit board.